Method for manufacturing incandescent lamps



Jan. 19, 1960 G. MEISTER ETAL 2,921,827

METHOD FOR MANUFACTURING INCANDESCENT LAMPS Original Filed Jan. 20, 19563 Sheets-Sheet 1 Q M ..-V 35. E INVENTORS @pokes 1s/:rre ...r s 5Aar/fours Fr cem/Lu'.

Jan. 19,' 1960 G. MEISTER l-:TAL 2,921,827

METHOD FOR MANUFACTURING INCANDESCENT LAMPS Original Filed Jan. 20, 19563 Sheets-Sheet 2 INVENTOR area/ece Mi/.s-rsz a NUMA/i5 E' cra-20AM.

, I @QQ-13%? Jan. 19, 1960 G. MEISTER ETAL METHOD FOR MANUFACTURINGINCANDESCEMT LAMPS Filed Jan. 20, 1956 3 Sheets-Sheet 3 Originalffm/:Perma INVENTORJ:` G50/C65 IVE/57757? .nl Bly/fawn.; E' Cnel/LuMETHOD FOR MANUFAtCfllURINGV INCANDES- 'CENT LS George Meister, Newark,and Nicholas lF. Cernlli, Caldwell, NJ., assignors to WestinghouseElectric Corporation, East Pittsburgh, Pa., a corporation ofPennsylvania f Original application January 20, 1955, Serial No.560,441. Divided and this application September 30, 1957, Serial No.692,953

4 claims.` (ci. sis-12) This invention relates to incandescent lampsand, more particularly, to diffusing coatings for incandescent lampenvelopes and to a process for applying a diiusing coating to anincandescent lamp envelope and is a division of application Serial No.560,441, filed lan. 20, 1956, entitled Incandescent Lamp whichapplication is a continuation-in-part of application Serial No. 444,316,led July 19, 1954, now abandoned, titled Incandescent Lamp with LightDiffusing Coating and Method of Manufacture by the co-inventors herein'.

v Heretofore commercially-available incandescent lamps with atinely-divided, light-diiusing envelope coating have had a silicacoating applied to the lamp envelope by methods as outlined in U.S.Patent No. 2,545,896 to Pipkin, by flushing processes, `or as outlinedin U.S. Patent No. 2,661,438 to Shand. In the flush-coating processes ofthe prior art, finely-divided silicon dioxide (silica) is suspended in avolatile solvent such as butyl acetate with a'binder such asnitrocellulose to impart the desired coating viscosity. In suchhush-coating processes the silica must be maintained substantially freefrom moisture, or such moisture Will react with the nitrocellulosebinder which is Water insoluble and deleteriously affect the resultingcoating. The process for removing moisture from finely-divided silicabefore ilush coating has entailed baking the silica at relatively hightempera! tures, for example, 825 C., or higher, and, as will hereinafterbe explained, such baking of the silica deleteriously affects themaintenance of the nished or processed silicacoated lamps. A

In the silica-coated lamps prepared by the process of burningorgano-silicates to form a fume or smoke, as disclosed by Pipkin in hispatent, the resulting silica formed by the burning is quite inert withregard to moisture-repossessing characteristics.` Further, the cost oforgano-silicates is relatively high and the cost ofcommercially-available, finely-divided silica is roughly one quarterthat of the organo-silicate, which lower cost favors the flush-coatingmethods of the prior art which can use commercially-available silica.However, the solvents which are used in the flush method, butyl acetate,for example, are relatively expensive, which somewhat decreases the costadvantage realized through using a commercially-available silica in aflush-coating process. In the silica-coated lamps as prepared by theprocess of spraying onto a heated bulb an alkaline-reacting silicaaquasol carrying large silica particles, as disclosed in the Shandpatent, the silica coating is relatively inert to moisture-repossessingability, which apparently is attributable either to the structure of thesilica Vwhich results from the method of preparing the vsilica gel or tothe method ,of applying the coating to the bulb. Silica aquasols arealso relatively expensive as compared to commercially-available silicasince considerable processing isY required, and in addition silicaaquasols containing some large silica particles must be used inrelatively large amounts on the incandescent lamp enf,

Velope in order to achieve adequate light diffusion.

Sttes arent ice It is the general object of this invention to-provide amethod for applying a finely-divided, light-diiusing coating to anincandescent lamp envelope, which willl result in improved performancefor the completed lamp.

It is another object of this invention to provide. a method of making asilica-coated incandescent lamp which has improved lumen maintenance.

It is still another object to provide a methodY for" electrostaticallyapplying afinely-divided, light-diffusing material to an incandescentlamp envelope whereby the resulting lamp displays improvedlumen-maintenance.

The aforesaid objects of the invention, and other objects which willbecome apparent Vas the description proceeds, are achieved by providingamethod for coating light-diffusing silica onto an incandescent lampenvelope so that it acts as a moisture getter to improve the lumenmaintenance of the nished lamp.

For a` better understanding of the invention, referenceV should be hadto the accompanying drawings wherein: Fig. 1 is a graph of silicamoisture content vs. silica temperature for samples of silica powdercooled from various firing temperatures;

Fig. 2 is a graph representing silica moisture loss vs.`

firing temperature for a silica powder;

Fig. 3 represents `moisture repossessing abilities for Various types ofsilica substances;

Fig. 4 represents moisture gain under 100% relative humidity conditionsfor samples of silica powders initially red at various temperatures;

Fig. 5illustrates a silica-coated incandescent lamp;v

Fig.` 6 represents a' rst step in the electrostatic coatingl process;

Fig. 7 represents a coating-material smoke generatorg Fig. 8 illustratesthe coating operation for electrostatically applying the coatingmaterial to the lamp envelope;

Fig. 9 illustrates the bulb-lehring operation following the coatingoperation;

k Fig. 10 represents the sealing-iu operation following bulb lehring;

Fig. 11 is a graph representing observed brightness for various types ofsilica-coated envelopes vs. distance from neck to top of bulb, i.e., thecandle powerV distribution for silica-coated envelopes;

Commercially-available silica is normally prepared by precipitatingsilica from a silicate by means of an acid, for example by precipitatingsilica from sodium silicate by means of hydrochloric acid. Stich silicais substantially white, porous, generally amorphous and normallyinherently spherical in couiigurationvas far as the ultimate particlesare concerned. By the descriptive term, generally amorphous, it is meantthat terns do not showv sharply-dencd lines. Also, the porous nature ofthe silica is another way of stating that the ultimate particles areloosely packed. l

high moisture content which may vary from 6 to 15% by weight, forexample. However, the commercially-available silica should possess orhave at least 1.7% by Weight of moisture when heated to 200 an improvedlumen maintenance for the processed lamp. Whether the moisture in thesilica as received is `absorbed or adsorbed is not definite, but it isprobable that the moisture is possessed by the silica as both absorbedand adsorbed moisture. Of course, if the silica has been baked or firedduring processing, the moisture coutent may vary considerably. Thefollowing discussion of the moisturefpossessing and repossessingproperties of silica is based on test data performed on silica which hasnot been baked or tired by the commercial supplier during initialprocessing or preparation or in any other Way subjected to amoisture-removing process which renders the X-ray diraction pat-` C. inorder to 'provide l e i Y 2,921,827 Y y y silica relatively inert withrespect to moisture repossession, hereinafter explained.

Extensive tests have been conducted on commerciallyavailable silicawhich has armoisture content of about 12% by Weight, which V12%`represents an` average moisture content for such silicasf In order toVdetermine the totall moisture content of the finely-divided silica,asreceived, a sample is accurately weighed, then tired at about- 1000 C.until no more, weight loss is observed. The l000 C. tired sample is thenreweighed before it can regain any moisture from the atmosphere. Theweight difference in the silica before and after ring at 1000 C.representsthe moisture content of'the original sample and may be calledthe ,loss on ignition, as it is known in the art.

The moisture-possessing and repossessing characteristics of silica varevery unusual and extensive tests have been conductedV on thesecharacteristics, the results of which testsare graphically representedin Figs. 1, 2, 3 and 4, wherein moisture content is plotted'vs. thesilica-temperature with the initial silica tiring temperatures.y alsobeing indicated.` In conducting the tests which provided the accumulateddata asV represented in the curves of Figs. 1-4, silica samples wereheated to predetermined ternperatures, for example to 900 C., until aconstant weight for the 900 C. tired sample was obtained. This tiringtemperature of 900 C. islrepresented at point (a) in Fig. l. Thesamplewas weighed after Viiring and before it could acquire or repossessanyv moisture from the atmosphere. The sample. was then allowed to coolslowly under normal room conditions of temperature and humidity (25 C.and 30-50% relative humidity) and a gradual weight gain was observed asthe sample slowly cooled and acquired moisture from the atmosphere. Whenthe sample reached room temperature the test was stopped. The resultslof this 900 C. test are illustrated in the lowermost curve (Curve A) ofFigi and, as shown, the 900 C. tired sample regained a total weight ofonly 1.15%. Similar tests were conducted at firing temperatures of 625C., 515 C., 335 C., 230 C., 155- C., 100 C.,and 65 C. Ineach of theseVtests the silica powders were tirst maintained at the designated ringtem` peratures until no further weight losses from the samples wereobserved. The samples were then allowed to cool slowly at normal roomconditions, of temperatureA andv humidity until room temperature wasattained, whereupon the tests were stopped. The results. of these testsare also graphically represented in Fig. 1 by the curves B, C, aD, 55E,HF, G15 and 44H.

An analysis of the curves represented in Fig. 1 indicates that thehigher the tiring temperature for silica powder, the less the moisturewhichwill be resorbed as the silica cools down to room temperatures.Also, a silica powder which has been red until no additional weight lossis observed (i.e., when steady-state conditions are obtained) has theability to repossess only a certain limited amount of moisture. Forexample, a 515 C. fired powder on being cooled 'to room temperature canrepossess about 2.0% by weight of moisture. If the same 515 C. redpowder were cooled ronly to 110 C. in the absence of moisture, it wouldhave the ability to repossess 0.65% by weight of moisture. Referring topowders which are tired at other than 515 C., assuming that thev firedpowders are maintained under substantially moisture-free conditionsafter firing, the moisture-repossessing abilities at 110 C. are asfollows: 0.40% by weight for 9000 C. fired powder, 0.5% by weight for625 C. tired powder, 0.7% by weight for 335 C. tired powder, 0.65% byweight for 230 VC. red powder and 0.4% by weight for 155 C. red powder.it can thus be seen that as silica powder is iired at Vhigher and highertemperatures it tends to `lose its moisture-repossessing characteristicsandwhencooled to 110 C. the moisture repossession approaches a maximumfor silica powders fired at from about 230 C. to 515 C.

Thereis illustrated'in Fig. 2.a curve showing moisture loss vs. ringtemperatures for silica powders which .orig-V inally possessedapproximately 12% by weight of moisture. Ordinates on this curve. weredetermined by firing powders at the temperatures as indicated until nomore weight loss was observed V(i.e., when steady-state conditions werereached). As illustrated, the moisture loss generally' follows threestraight lines, from room temperature up to 200l C. firing temperatureit is theorized that the loss is primarily adsorbed moisture.. From 200C.

ability to repossess additional moisture, and indeed willA act as amoisture getter. At higher ringtemperatures,

however, the silica loses its moisture gettering characteristics.

Other materials which have been used to coat lampss to form diffusingcoatings do not exhibit these moisturerepossessing characteristics tosuch a degree. For example, there are illustrated in Fig. 3 curvesshowing the. moisture-repossessing abilities of sodium silicate, silicawhich is formed byY burning ethylY orthosilicate, a silica aquasolcontaining large silica particles, and for purposes of comparison, a 900C. fired silica powder, a 625 C.j

iired silica powder and a 500 C. fired silica powder. As illustrated incurve i of Fig. 3 when heated to 500 C. and then allo-wed to cool toroom temperature under normalV room conditions, silica formed by burningethyl silicate is relatively inert with regard to repossession ofmoisture, since such silica gains only 0.6% by weight of moisture.Sodium silicate when fired at 500 C'. and cooled under the sameconditions repossessed only 1.05%

of'weight by moisture, as illustrated in curve I of Fig.

3. An alkaline-reacting silica aquasol containing colloidal silicaadmixed with ya limited amount of larger Vsilica particles for purposesof light diffusion, and prepared as outlinedin the aforementioned Shandpatent, when tired at 500 C. and allowed to cool to room teml perature,will repossess only 1.2% by weight of moisture,

las illustrated in'curve K of Fig. 3.

A 900 C. tired silica powder, when later red at 500 C. and allowed f 'tocool to room temperature will repossess only 1.05%

by weight of moisture as shown in curve I of Fig. 3,'

i.e., itdisplays the same characteristics as sodium silicate. A 500 C.ired silica powder (curve-L) and a 625 C.

tired silica powder'(curve l when retired at 500 C. and allowed to coolat room temperature will respectively gain 2.0% and 1.3%-by weightfofmoisture.

The inertness of the silica formed Vby burning ethyl silicate isattributed tothe very high flame temperature;V

of ethyl silicate (1320 kilocalories per mole liberated to form S102,CO2 and H2O) and'it has been shownthatv the higher the temperature towhichsilica- `is exposed the more inert with regard to'moisturerepossession' itv The inertness of sodiumy silicatek with regardYbecomes. to moisture-repossession ability is .attributedtothe fact thatsodium silicate is an entirely different compound from silica, andcannot be expected to display the same` physical properties. Therelative inertness with respect to moisture repossession of theadmixture of alkaline.- reacting silica aquasol and large silicaparticles is at-y tributed either to the silica-gelV structure, or tothe method. of applying thecoating wherein the gelhas an apparent.tendency to frit itself 'when-sprayed onto ahot bulb,-as

outlined in the aforementioned Shand patent. The relative inertness ofthe 900 C. fired powder withrespec't to moisture repossessiony isattributed to a basic structural change in the highly-tired,finely-divided silica pow-Y;

' ty for two hours and the illustrated, a 900 der, which structuralchange is not definitely understood.

Even when a 900 C. tired silica powder is allowed to remain at normalroom temperatures and humidity for for periods of months, it willrepossess very little moisture.

As a further illustration of the moisture-repossessing characteristicsofl finely-divided Silica, various fired samples were allowed to cool toroom temperature while maintained under substantially moisture-freeconditions. These fired samples were then exposed to 100% humidipercentmoisture gain in weight was measured. This percent moisture gain inweight is plotted vs. initial tiring temperatures in Fig. 4 and, as C.tired powder gains only about 0.7% by weight and the 150 C. fired powdergains 14.5% by weight of moisture. It should be noted that on tiringsilica, substantially all moisture is driven off at about 825 C. andtiring at higher temperatures does not result in any further moistureloss. However, tiring at temperatures in excess of 825 C. does renderthe silica more inert to repossession of moisture and this isillustrated in Fig. 4 where the 1000 is shown to be more inert towardmoisture repossession than the 825 C. tired powder.

It is deemed proper to note that ever since the first days of theincandescent lamp, engineers have resorted to all conceivable means andmechanisms to remove all possible moisture from the finished lamp. Thisis because any moisture present in the nished lamp tends to set up thewell-known, so-called water cycle with the tungsten filament during lampoperation. In this water cycle, the moisture reacts with the hottungsten filament to form tungsten oxides and release atomic hydrogen.The tungsten oxides deposits on the envelope surface. The atomichydrogen reacts with oxygen present in the deposited tungsten oxides toform a black tungsten deposit on the envelope and more water vapor, andso on, until the envelope is quite blackened and relatively opaque. Thisproblem has existed with acid-etched bulbs, as well as silica-coatedbulbs.

4It has been found that by using a silica coating in accordance with theteachings of this invention, there is provided a diffusing coating whichalso acts as a moisture getter and which will inhibit theheretoforementioned water cycle. Thus, the lumen maintenance undernormal-operating conditions for the silica-coated lamps of thisinvention is measurably improved over the lumen-maintenance undernormal-operating conditions for all lamps of the prior art. The priorart has taught that all possible moisture should be removed from theprocessed lamp in order to produce the best possible normal-operationlumen-maintenance. In this case, however, removal of all moisture fromthesilica impairs its moisture-gettering action. v

Silica-coated lamps have been prepared under the same conditions exceptthat some of the lamps were coated with 900 C. tired silica powder andother lamps were coated with 600 C. tired silica powder, both powderrings being prior to the envelope coating step. Of course, after firingat the designated temperatures and during lamp processing, the tiredpowders were maintained as moisture-free as possible. The 600 C. firedsilica coated lamps had a 70% normal-life lumen-maintenance which wasappreciablygreater than the 70% normal-life lumen-maintenance of thelamps coated with the 900 C. red powder.. It is significant to note thatall lamps which were coated with the two powders were prepared under thesame controlled conditions and the maintenance improvement can thus beattributed solely to the fact that the powders were processed by firingat different temperatures before beingl coated on the lamp envelope. r

ln explanation of the term 70% normal-life lumenmaintenance, whichtermis well-known through the lamp art, it is .generally accepted thatthe C. fired powder lumen output of a lamp, when measured at 70% of itsnormal life, is an accurate indication of the performance which is to beexpected throughout the life of the lamp. In control tests fordetermining the 70%. normal-life lumenmaintenance, the initial lumenoutput of the, lamp is corrected for any variations in actual lamp lifevfrom the rated lamp life, as is customary in the lamp art.

to the 900 C. silica,

The foregoing tests, and all normal-life tests herein referred to, wereconducted on watt lamps burned in` a base-upward position,l which is theusual service operating position for 100 watt lamps. The bulbtemperatures forsuch lamps when burned in such al position varyconsiderably from one portion of the bulb to another, but the minimumbulb temperature as measured with a` pre-heated thermocouple isapproximately C. 'In the following discussion the normal-operationminimum envelope temperatures will be considered, since Ithe silicacoating at the coolest portion of the lamp envelope has the greatest,moisture-repossessing ability,4

which moisture-repossessing` ability results in the improved normal-lifelumen-maintenance, as will .be fulther discussed.

Refering now to the curvesillustrated in Fig. 1, where the 900 C. firedpowder is coated onto a bulb it must necessarily be vsubjected totemperatures during lamp processing which are suiiicently below thedeformation temperature of the soft glass envelope so that the envelopewill not be damaged during processing. Assuming the coated lamp envelopeis baked or lehred at about 450 C. during processing (this lehringtemperature was employed in processing the lamps), a 900 C. redv powderwill have the opportunity to gain or repossess approximately 0.15% by'weight of moisture (e.g., note the differences in valuesof moisturecontent between the ordinates of points (a) and (b) on the curve A ofFig. l, wherein a 900 C. iired powder on cooling to 450 C. can acquireabout 0.15% by weight of moisture). On further cooling to 110 C. a 900C. red powder will have the ability to accumulate or repossess anadditional 0.25% by weight of moisture (i.e., the vditference inordinate values of points (b) and (c) on the 900 C. fired powder curveA. A 600 C. `fired powder, in contrast, on cooling from 450 C. to 110 C.has the ability to accumulate an additional 0.45% by weight of moisture(eg. note the ordinate differences between points (d) and (e) obtainedby extrapolation between the 625 C. and 515 C. tired powder curves).Since the lumen-maintenance for the 600 C. silica coated vlamps areappreciably better than the lumen maintenance for the 900 C. silicacoated lamps, which lamps were processed the same except for the silicapowder, it would seem to follow ipso facto that the additional moisturerepossessing ability of the 600 C. silica, as` compared is responsiblefor the improved lamp lumen-maintenance characteristics.

It should be noted :that the tenacity with which silica getters andholds small amounts of moisture'apparently increases greatly as thesilica becomes more moisture hungry. Thus, a relatively small increasein moisture gettering ability, expressed as a percent by weight,represents a very large increase in the tenacity with which silicagetters and holds moisture.

It has been found that in order to have a 70% no1'- mal-lifelumen-maintenance which is appreciably better than the correspondingnormal-life lumen-maintenance of prior art lamps, the silica coatingmust have a moisture content which is at least equivalent .to themoisture content o-f a 625 C. fired silica powder which has the abilityto repossess an additional 0.4% by weight of moisture at minimum lampenvelope operating temperatures. The total moisture content fora 625 C.tired powder, before such powder is allowed to repossess any moisture isabout 1.4% by weight. At 450 C., under normal room conditions, the totalmoisture content for this 625 C. red powder 'will be about 1.55% byweight and if the coated 7 envelope were lehred during lamp processingtoabout 450 C., under normal-room conditions, the totalmoisture contentfor this 625 C. fired powder would thus' be about 1.55% by weight.Assuming the lamp is tipped-off while the moisture content of 625 C.silica is maintained at not greaterthan 1.55% the silica will stillpossess an ability to accumulate an additional 0.4% by weight ofmoisture at 110 C., which is the normaloperation minimum envelopetemperature for a 100 watt lamp, and which 0.4% represents the moisturegettering abilities of the coated 625 C. tired powder when processedinto a finished lamp. When higher, fired silicas are processed into afinished lamp their potential moisture gettering abilities are solimited as to minimize any maintenance improvement which may be realizedthrough the so-called silica moisture gettering action. Since thismoisture ygettering action results in improved 70% normal-lifelumen-maintenance, the silica coatingmust have at least 1.5 by weight ofmoisture and the ability to repossess an additional 0.4% by weight ofmoisture at normal-operation minimum envelope temperatures. Thisestablishes a lower range for moisture content in the silica coating andthe minimum moisture gettering ability which the silica coating mustpossess.

At the upper permissible moisture content limitation for the silicacoating it would seem possible to use any silica which has beensubjected just prior to lamp tip-off to temperatures which arereasonably in excess of the normal-operation minimum lamp envelopetemperature of 110 C. For example, the lamp could be baked just prior toexhaust and tip-off at a temperature of 200 C., which would result in amoisture gettering ability for the silica coating of approximately 0.5%by weight at normal-operation minimum envelope temperatures (this figureis obtained by extrapolation between the 230 C. and 155 C. curvesdesignated E and F in Fig. l1). This of course does not take intoaccount the operation of lamps in enclosed or recessed type fixtureswhere the minimum lamp envelope temperature vmay be as high as 225 C. Ifa coated 200 C. fired powder, having a moisture content of approximately4.8% was exposed to a temperature of about 225 C., as in a recessedxture, the coating of 200 C. fired silica would havey the ability togive olf approximately 0.3% by weight ofmoisture,

resulting in some lamp-blackening and decreased lumen maintenance forsuch applications. However, silicacoated lamps are now soldV at apremium price and are normally intended to be used in fixtures wheretheir esthetic, even appearance will be visible. Thus silicacoated lampshaving a moisture content as high as 4.8% will still be very acceptablefor normal operation -where the minimum envelope temperature is 110 C.,provided such silica also has the ability to repossess at least 0.4% byweight of additional moisture. Such lamps will thus show an increased70% normal-life lumen-maintenance over the silica coated lamps of theprior ait and this normal-life lumen-maintenance improvement will morethan offset any increased blackening encountered inrecessed or otherhot-fixture applications.

It can thus be stated that in order to show an appreciable improvementin 70% normal-life lumen-maintenance, the silica coating must possessatleast 1.55 by weight ofmoisture and vnot 'more than 4.8% by weight ofmoisture, and in addition must have the. ability to possess at least0.4% Vby weight of additional moisture at normal-operation minimum lampenvelope temperatures.

If it is desired to process the lamp so that itsperfformance inspecial-type recessed or other hot fixtures will be at least as good asthe performance of the inside-frost lamps of the prior art, it isnecessary to limit the moisture content of the silica coating to notmore than 4.0% by weight, or otherwise expressed, the moisture contentof the silica coating should be equivalent to the moisture content ofabout a 300,o C. fired silica (extrapolating between curves D and E7 ofFig. 1). Of course the 8T silica should still have the abilityrtorepossessv at least 0.4% by weight of additional moisture atnormaloperation minimum lamp envelope temperatures if the lumenmaintenance is to be improvedoverA the lamps of the prior art.

Following is a table, designated Table i, inwhichare listed 70%normal-life, lumen-maintenance figures for types of lamps which arecommercially available and` for a representative silica-coatedlampofthis invention.

Lamp type 70% normal-life maintenance (indicated as a percentage of theinitial Y vlumensper watt).

Inside-frost type bulb '93.8.

Silica-coated bulb (formed by burning ethyl orthosilicate) 93.7.

Silica-coated bulb (900 C. red

powder coated by flush) 92.7.

Silica-coated bulb (silica aquasol-large silica particle mixture sprayedo n hot bulb.) 92.0. Silica-coated bulb (450 C. fired powder) 95.2.

An electrostatic process'is preferable for coating onto abulbnely-divided silica which possessesV limited and controlled amountsof moisture and an apparatus for electrostatically coating silica ontoanenvel'ope and further. processing the coated envelope is illustratedin Figs. 6 9.

As heretofore noted, silica possessing substantial amounts of moisturecannot be coated onto a bulb by the flush methods of the prior art sincethe organic binders which are necessary to impart the desired viscosityto the coating composition are not water soluble vand are deleteriouslyaffected by any appreciable amounts of water in thesilica. Thus whereliush methods are used to coat lamps, 825 C. to 1G00 C. fired powdershould be used to substantially eliminate the moisture possessed by thesilica.Y Y Y i in Fig. 5 is illustrated a silica-coated incandescentlamp 20 comprising a vitreous, light-transmitting envelope 22 carryingan internal coating of moisture-containing, finelydivided silica-24 andhaving a mount sealed to the neck thereof. A brass or aluminumscrew-type base 26 is cementedY to the neck to facilitate connection toa power source, as is usual. As is well known, the mount comprises avitreous re-entrant stem Vpress 2S having leadin conductors 30 and 32sealed there-through and supporting a refractory metal filament 34,`suchas tungsten, between their inwardly extending extremities. The envelopepreferably contains inert gases suchV as nitrogen, argon, krypton, etc.,or mixtures thereof, as is well-known, although the lamp may be a vacuumtype, if desired.

In electrostatically applying the diffusing silica coating to the innersurface of the unsealed bulb, the open-necked bulb is first placed underand supported by a hollow lava chuck 36, as illustrated in Fig. 6, whichchuck cooperates with the bulb cullet 38 and bulb neck 40 to support thebulb. While thus supported, the bulb is rotated either manually or by abelt drive (drive unit not shown) and heated by gas-air burners'42 toapproximately 100 C.` Because of the negative temperature coefficient ofelectrical resistance or glass, this heating renders the envelopesubstantially uniformly electrically-conductive. The heating temperatureof C. is given only by way of example and not by way of limitation sincethe temperature to which the glass is heated` to render it substantiallyuniformly electrically-conductive is not particularly critical and maybe varied considerably, according to .the type of glass beingheated,vfor example, temperature extremes of 70 C. to 300 C. have beenused, although these temperatures are not intended to be limiting. Itshould be noted that most incandescent lamp bulbs are fabricated of thewell-known lime glass.

' during the heating and later steps of the process.

There is illustrated in Fig. 7 a smoke generator unit 44 for producing asmoke of finely-divided particles suspended in air, prior toelectrostatic deposition of the powder. The smoke generator comprisesgenerally a powder and smoke reservoir 46 having an outlet 48 at thebottom thereof through which the finely-divided material is admittedinto a mixing venturi S9. Compressed air is admitted to the venturithrough a pressure-regulating Vvalve 52 and thence to the venturi wherethe finely-divided material is picked up and 'carried through a feedconduit 54 to cause the air-particle mixture to impinge upon a target 56to break-up agglomerates which might have formed and to dispersethoroughly the coating material to form a smoke of finely-dividedparticles suspended in the air vehicle.

The powder before being placed in the smoke generator unit must befinely-divided and may be ground in an air-velocity type grinder such asmarketed under the trademark Micronizer by Sturtevant Mill Co., Boston,

Mass., or as marketed under the trademark Wheeler Mill by C. H. WheelerMfg. Co., Philadelphia, Pa. This breaks up the overly-large particleagglomerates. For the specific embodiment of the particle smoke nozzle,which Will hereinafter be illustrated and described, it Vis preferableto maintain a particle-smoke pressure within the reservoir 46 between 6and l2 pounds during coating to cause the particle-smoke to pass throughthe smoke nozzle at a desirable velocity. To maintain the smoke pressurein the reservoir within these aforementioned preferred pressurelimitations, an indicating gauge 58 is provided from which the operatormay have a visual indication so that the pressure-regulating valve 52may be manually adjusted to maintain the smoke pressure in the reservoirwithin'the aforementioned preferred pressure limitations. Suchpressure-indicating and pressureregulating valves are well-known. Asmoke-nozzle conduit 60 connects the smoke reservoir with the injectornozzle assembly 62, as shown in Fig. 8. lTo control the flow of smoke tothe nozzle assembly, a manually-operable butterfly valve 64 is providedin the conduit 60.

The air compressor 66 which supplies air to the smokegenerator unit ispreferably regulable between 2 lbs. and 20 lbs. output pressure and anair dryer 68, such as a well-known, aluminum-oxide type air dryer isprovided in the output line of the compressor so that the particlepowder may be maintained under substantially watervapor-free conditionsuntil it is forced into the uncoated bulb. A power-driven agitator 70(power source not` shown) is provided near the base of the reservoir toagitate continually the finely-divided coating material to keep it in afinely-divided state by breaking up the largest particle agglomerates.

. -In order to control better the moisture content ofcommercially-available silica used in coating, the powder may be baked,if desired, before coating, although the moisture possessed by thesilica may be carefully controlled by lehring or baking the coated bulbsafter the coating operation and before sealing in and/r exhaust. Theonly limitation to powder baking before coating is that the bakingtemperature should not exceed 625 C. so as to impair the potentialmoisture-gettering properties of the silica as processed into the lamps,as heretofore explained.

In Fig. 8 is shown the coating operation for the bulb. The positive pole72 of a high-tension, direct-current source is electrically connected tothe gas-burner unit 42 and the negative pole 74 is electricallyconnected to a probe 76 which projects through the hollow-lava chuck 36into the lower extremities of the bulb neck. If desired these polaritiesmay be reversed with but little ef- The particle smoke injector nozzleassembly 62 is circumferentially disposed. about the probe 76, and thenozzle assembly connects with the nozzle conduit 60 of the smokegenerator. The air which is present in the bulb and the particle smokewhich does not deposit on the bulb wall during coating passes through areturn conduit 78 which is disposed about thenozzle assembly 62 and thenozzle conduit 60 and which discharges into a collecting hopper (notshown) so as to collect the uncoated particles for reprocessing andfurther use. A conduit support collar 80 supports the probeandnozzle-conduit assemblies and may be positioned longitudinally withrespect to the lava chuck and bulb neck either manually orautomatically.

It has been found that the particle smoke must be forced through thenozzles of the nozzle assembly in order to coat properly for when thenozzle assembly is removed and the particle smoke is passed through thenozzle conduit into the heated bulb an imperfect coating results. It isthus theorized that the particles when passing through the nozzles arefrictionally charged and the application of the high voltage merelyattracts the particles carrying an opposite charge to the bulb wallWhere the charge on the particles is dropped. In support of this, testswere conducted with silica where the applied uni-directional potentialwas varied between 8 kv. and 25 kv. vwith noobserved dierence in thevamount of silica which was deposited (i.e., the coating weight on thebulb wall) at the extremes of the 8 kv. to 25 kv. applied Voltage. Also,only about 50% of the silica powder injected through the nozzle into thebulb is actually coated, the other 50% passing through the returnconduit 78.

When the silica smoke is forced through the nozzles, the particles carrya small net-negative charge, i.e. there areA more `negatively-chargedparticles than positivelycharged particles. This is perhaps due to thestainless steel of which the nozzle is fabricated and apparently this isresponsible for a slightly better powder deposition on the bulb when thebulb wall is made positive with respect to the bulb wall represents ageneral departure in mechanism from that mechanism which is generallyaccepted as customary in the electrical-precipitation art. Inexplanation of this, in the usual electricalv precipitation theparticles are charged by means of gaseous ions or electrons as opposedVto a static-frictional charging through turbulence. The chargedparticles are then transported to the collecting electrode by the forceexerted on the particlesy by the electric eld, and the charged particlesare then discharged at the collecting electrode. Thus the electrostaticdeposition of this invention apparently differs from the conventionalelectrical precipitation in the particle charging means whereinsubstantially all particles within the lield are deposited.

A s a specific exampleforsilica coating a bulb designed for a W. lamp,the nozzle-injector assembly has eight, evenly spaced nozzles,circumferentially disposed about the probe and each having a diameter of46 mils. As heretofore noted, the preferred pressure in the smokegenerator may vary between 6 and l2 pounds. In coating a bulb adaptedfor 100 watt operation, the butterfly valve 64 may be opened for about 2seconds while applying a high tension D.C. of l5 kv. between the bulbwall and the` probe. This will deposit approximately 50 mg. of silicaonto the bulb. In coating bulb sizes other than the size adapted for 100watt operation, the number of nozzles which may be used may varydepending on the bulb size. Alternatively, fewer nozzles having a largerdiameter or morey nozzles having a smaller diameter may be used in thenozzle assembly to coat identical bulbs in order to achieve the samecoating result. The nozzle assembly thus constitutes a diffusing orificewhich projects charged particles into the heated bulb.

After being coated, the bulb is baked or lehred while rotated on thelava chuck, as illustrated in Fig. 9. Bulb lehring is necessary to driveofi moisture which may have accumulated during coating and to render thesilica coating as moisture possessive or acquisitive as possible. Thelehring maybe accomplished by gas-air burners d2, as illustrated, andthe lehring temperatures may vary considerably depending on the priorprocessing of the silica coating powder and the conditions under whichthe processed lamp is intended to operate. For example, if a silicapowder is tired before the coating operation at a temperature of about500 C. fora sufficient time to approachV steady-state conditions withregardto moisture content, a bulb lehr of 350 C. for a period of l0 to20 seconds will normally he suiiicient for the silica coating to havesuicient affinity for moisture to provide an improved lumen-maintenanceat normal-operation minimum envelope temperatures, provided the mount issealed-in, lamp exhausted, gas-fill inserted and exhaust tube tippedolfwhile the bulb is still hot, thus Vpreventing the silica coating fromrepossessing appreciableV amounts of moisture from the atmospherebetween the coating, sealing-in and tipping-olf operations.

In order to insure adequate moisture-free conditions for the silicacoating, particularly where it is desired to operate under hightemperature conditions, it is desirable to lehr the coated bulb at from400 C. to 500 C. for about l to 20 seconds and even Iat this lehringtemperature range it is desirable to simultaneously ush the coated lampwith hot, dry air, or other gas at a temperature of about 250 C., forexample, to carry away all possible moisture. The air flush temperatureis not particularly critical and may vary from about 150 C. to thelehring temperature of the bulb.

If the silica powder has not been baked or tired before the coatingoperation, higher lehring temperatures preferably are used whilesimultaneously flushing the bulb with hot dry air. For example, a bakeor lehr of 450 C. for about l seconds is not considered excessive wherea relatively moist, unired, silica coating must be activated to impartthereto adequate moisture gettering abilityfor normal lamp operation. lImmediately following the lehring operationand while the bulb is stillhot, the mount is sealed in as illustrated in Fig. ,102, It is desirableto flush the bulb with hot, dry nitrogen, or other inert gas, whilesealing lthe mount to the bulb neck in order to remove any moisturewhich may accumulate from the sealing fires, which are normally providedby gas-air burners, as is usual. Such hot, drynitrogen flushing ispreferably accomplished through the exhaust tube 80 .in order tomaintain a slight pressure within the bulb to force any moisture'outofthe neck.

Immediately following the sealing-in operation, andl While the bulbportion of the envelope is still hot, the

lamp is exhausted through the exhaust tube, the'gas-lill f inserted andthe exhaust tube tipped-olf, as is-customary. It may be desirable tofurther bake the bulb on exhaust to insure thatall possible moisture isremoved to give the silica coating all possible moisturegetteringability. Baking on exhaust is not absolutely necessary, but isdesirable, particularly where the processed lamp is to be operated inhot-recessed or rother .high-temperature-type fixtures. Y

After neck andl the lead-in conductors vconnected by vwellknown lampbasing techniques (basing operation Anot shown). Y

tipping-off, the lamp base is cemented to thev VLamp type:

The main purpose of a lamp envelope diffusing coating is to diffuse andsoften the light emitted by the incandescent filament. Thus theperformance of any diffusing coating can be evaluated by the amount ofdiffusion efected as compared to the percentage of light which istransmitted through the coated, light-diiusing envelope.

the observed brightness measurements represented in Fig. ll wereintegrated over a circular area of the lamp surface approximatelyV 0.1"in diameter (roughly 0.008 sq. in). it is obvious that by increasing theintegrated area, hotspots will be smoothed out in the observedbrightness measurements, or vice-versa, by decreasing the integratedarea, hot spots can be intensified.V VrThus whenever brightnessmeasurements, as illustrated in Fig. ll, are to be evaluated, the areaover which such measurements are integratedV should be indicated inorder that the observed data may be given the proper evaluation. Thecurves of Fig. ll are identified as follows. Curve N represents astandard acid-etch, inside-frost bulb.

Curve O represents an electrostatically-deposited silica coating on anacid-etched, inside-frost bulb. Curve P` represents an electrostaticallydeposited silica coating on a clear-glass bulb. Curve Q represents aburned ethyl orthosilicate silica coating on an acid-etched, insidefrostbulb. Curve R represents a silica coating on an acid-etched,inside-frost bulb, which coating is applied by spraying onto a hot bulba silica aquasol containing large particles of silica. Curve Srepresents a silica coating ,on an acid-etched, inside-frost bulb, whichcoating is applied by ilushing a 900 C. tired silica powder onto thebulb. As observed, Ithe general shape of all of these curves, with theexception of the inside-frost bulb which is shown for purposes ofcomparison, is substantially similar and the maximum observed brightnessfor each type.

of coated lamp is given in the following table; designated Table Il,

Table II Maximum brightness in candles per sq. cm. Acid-etched,inside-frost bulb 28.0 Electrostatically-applied silica coating oninside-frost bulb 7.4 Electrostatically-applied silica coating onclearglass bulb 7.2 Burned ethyl orthosilicate coating of silica oninside-frost bulb 7.3 Silica aquasol containing large silica particlessprayed onto hot inside-frost bulb 7.8 Flush coating of 900"Y C. redsilica on insidefrost bulb 8.2

It will be observed that the maximum brightness for theelectrostatically-applied silica coatings, both on clear.

and inside-frost type bulbs, and the maximum brightness for the burnedethyl orthosilicate coatings when applied to inside-frost bulbs areequivalent. rifhe maximum brightness for the flush-coated lamps iscomparatively greater, i.e., the diffusion effected by such coatings issomewhat. less. The maximum brightness for the silica aquasol-largesilica particle coatings on insidefrost bulbs light-transmission eciencyfor these coatings were tested and the results are given in thefollowing table. In conducting the coating-transmission-eiiciency tests,open-necked bulbs were placed over a standard light source in aphotometry sphere. A sensitive, linearresponsive photocell, shieldedfrom direct radiation from the standard source, indicated thetransmitted-light intensity, which indication is relative to theintensity of the standard source. For example, the standard light sourcewas energized-within the sphere and the photocell output noted. Thisreading represents the 100% value. The bulb whose transmission eiiciencywas to be measured was placed over the standard light source and thephotocell output measurement noted. This photocell measurement was thencorrected to indicate a percentage reading as compared to the intensityof the standard light source. The transmission efiiciencies for theheretofore-discussed various types of envelope diiiusing coatings wereas indicated below in Table III. Also indicated for purposes ofcomparison are the transmission Veiiiciencies for clear-glass bulbs andinside-frost bulbs.

Table III Transmission efciency Lamp bulb type: in percent Standardlight source 100 Clear-glass bulb 99 Acid-etch, inside-frost bulb 98.9Electrostatically-applied silica coating on clear- Y glass bulb 97.2

Electrostatically-applied silica coating on inside-frost bulb 97.2

Burned ethyl orthosilicate on inside-frost bulb 96.3 Silicaaquasol-large silica particles sprayed on hot inside-frost bulb 95.5 900C. fired silica powder ushed on insidefrost bulb 96.7

It will be observed that as compared to a standard acid-etched,inside-frost bulb, the electrostatically-applied silica coatings have atransmission eiliciency which is 1.7% lower. An equivalent burned vethylorthosxlicate coating on an inside-frost bulb has a 4transmissionefciency which is 2.6% lower than an inside-frost type bulb. Anequivalent silica aquasol-large silica particle, hot-bulb-sprayedcoating has a transmission eiciency which is 3.4% lower than a standardinside-frost bulb. Thus for an equivalent diffusion (see the foregoingtables listing maximum observed brightness) an electrostatically-appliedsilica coating has a transmission etciency which more nearly approachesa standard acid-etch, inside-frost type bulb than the silica-coatedbulbs of the prior art.

It will be recognized that the objects of the invention have beenachieved by the provision of a method for coating light-diffusing silicaonto an incandescent lamp envelope so that the coated silica also actsas a moisture getter to improve the lumen maintenance of the finishedlamp.

As a possible alternative embodiment, the electrostatically-appliedsilica may be steamed to increase adherence. Such steamed silicacoatings can still be made to act as a moisture getter with the properprocessing. In this case, a slightly longer and hotter bulb lehr isrequired to render the silica moisture hungry, e.g., a 475 C. lehr forabout 15 seconds. Also, the electrostaticallyapplied silica may be necksteamed, that is, only the neck of the envelope may be steamed toincrease adherence of the silica coating for the glass at this point,since imperfections arising from insuicient adherence .most often'oceurat the neck of t the lamp.

While in accordance with the patent statutes, one best embodiment hasbeen illustrated and described in detail, it is to be particularlyunderstood that the invention is not limited thereto or thereby.

We claim:

l. The process of applying finely-divided, porous, generally-amorphoussilica to an incandescent lamp envelope having an open neck, comprisingevenly applying said silica to the interior surface of said envelope,limiting the temperatures to which said Silica is subjected before,during, and after coating to not morev than 625 C., lehring saidsilica-coated envelope so that at normaloperation minimum lamp envelopetemperatures said silica coating will constitute a getter for at least0.4% by Weight of additional moisture, and hermetically sealing a mountto said neck and tipping-01T while maintaining said silica coatingwithin the aforestated moisture gettering condition.

2. The process of applying finely-divided, porous, generally-amorphoussilica to an incandescent lamp envelope havingan open neck, comprisingevenly applying said silica to the interior surface of said envelope,limiting the temperatures to which said silica is subjected before,during and after coating to not more than 625 C., lehring said coatedenvelope so that said silica coating will have from 1.55% to 4.8% byweight of moisture and so that at normal-operation minimum lamp envelopetemperatures said silica coating will constitute a getter for at least0.4% by weight of additional moisture, and hermetically sealing a mountto said neck and tipping-olf while maintaining said silica coatingwithin the aforestated moisture limitations and moisture getteringcondition.

3. The process of applying nely-divided, porous, generally-amorphoussilica to an incandescent lamp envelope having an open neck, comprisingelectrostatically applying said silica to the interior surface of saidenvelope, limiting the temperatures to which said silica is subjectedboth before, during and after coating to not more than 625 C., lehringsaid coated envelope so that said silica coating will have Ifrom 1.55%to 4.8% by weight of moisture andV so that at normal-operation minimumlamp envelope temperatures said silica coating will constitute a getterfor at least 0.4% by weight of additional moisture, and hermeticallysealing a mount to said neck and tipping-off while maintaining saidsilica coating within the aforestated moisture limitations and moisturegettering condition.

4. The process of applying finely-divided, porous, generally-amorphoussilica to an incandescent lamp envelope having an open neck, comprisingelectrostatically applying said silica to the interior surface of saidenvelope, limiting the temperatures to which said silica is subjectedboth before, during and after coating to not more than 625 C., lehringsaid coated envelope so that said silica coating will have from 1.55% to4.0% by weight of moisture and so that at normal-operation minimum lampenvelope temperatures said silica coating will constitute a getter forat least 0.4% by weight of additional moisture, and hermetically sealinga mount to said neck and tipping-off while maintaining said silicacoating within the aforestated moisture limitations and moisturegettering condition.

References Cited in the le of this patent UNITED STATES PATENTS2,521,642 Massa Sept. 5, 1950 2,538,562 Gustin et al. Jan. 16, 19512,699,371 Meister Jan. 11, 1955 2,811,131 Lopenski et al.` Oct. 29, 1957

1. THE PROCESS OF APPLYING FINELY-DIVIDED, POROUS, GENERALLY-AMORPHOUSSILICA TO AN INCANDESCENT LAMP ENVELOPE HAVING AN OPEN NECK, COMPRISINGEVENLY APPLYING SAID SILICA TO THE INTERIOR SURFACE OF SAID ENVELOPE,LIMITING THE TEMPERATURES TO WHICH SAID SILICA IS SUBJECTED BEFORE,DURING, AND AFTER COATING TO NOT MORE THAN 625* C., LEHRING SAIDSILICA-COATED ENVELOPE SO THAT AT NORMALOPERATION MINIMUM LAMP ENVELOPETEMPERATURES SAID SILICA COATING WILL CONSTITUTE A GETTER FOR AT LEAST0.4% BY WEIGHT OF ADDITIONAL MOISTURE, AND HERMETICALLY SEALING A MOUNTTO SAID NECK AND TIPPING-OFF WHILE MAINTAINING SAID SILICA COATINGWITHIN THE AFORESTATED MOISTURE GETTERING CONDITION.